Packaging for food, beverages and in particular beer and fruit juices, cosmetics, medicines, and the like are sensitive to oxygen exposure and require high barrier properties to oxygen and carbon dioxide to preserve the freshness of the package contents and avoid changes in flavor, texture and color. Blends containing small amounts of high barrier polyamides, such as poly(m-xylylene adipamide), typically known commercially as MXD6, with polyesters such as poly(ethylene terephthalate), PET, enhance the passive barrier properties of PET.
To further reduce the entry of oxygen into the contents of the package, small amounts of transition metal salts, such as cobalt salts, can be added to the blend of PET and polyamide to catalyze and actively promote the oxidation of the polyamide polymer, thereby further enhancing the oxygen barrier characteristics of the package. The use of such active oxygen scavengers, which chemically remove oxygen migrating through the walls of the package, can be a very effective method to reduce the oxygen transmission rates of plastics used in packaging. While currently available scavengers have found some utility, they also suffer from a variety of drawbacks that include lengthy induction periods before full activity is achieved and/or life spans (capacities) which are too short. In some instances, these deficiencies can be partially addressed by increasing the level of oxygen scavenger in the package structure. However, this typically increases the cost of the final package and produces undesirable effects on the appearance of the package, such as adding haze or color. In addition, increasing the concentration of the oxygen scavenger can complicate manufacture and recycling of the package. Thus, there is a need for improved oxygen scavenging materials that rapidly achieve high scavenging rates.
Transition metal salts have been added to PET polymers and to blends of PET polymers with polyamide polymers to impart active oxygen scavenging activity. Typical methods for incorporating these metal salts into the PET composition include feeding the metal contained in a liquid carrier into an extruder along with a feed of bulk PET pellets. Alternatively, a metal such as cobalt is frequently added to a melt phase process for the production of PET, such that the PET pellets already contain cobalt when fed to the extruder. In this method, the metal salts can be added in low concentrations corresponding to the desired concentration in the article, or in higher concentrations to form a masterbatch. However, adding metal salts to a melt phase process for making the polymer may result in discoloration or the generation of excessive levels of other undesirable byproducts such as diethylene glycol and acetaldehyde at the high temperature conditions and long residence times employed in a PET polymerization reactor. This condition is exacerbated if the metal is added early or the residence time of the polymer melt containing the transition metal is lengthy.
We have found that deficiencies in the oxygen scavenging activity are partly attributable to the form in which the transition metal is added to PET. We have also found that when cobalt is added to a bulk polyester polymer in a form of a solid concentrate comprising a polyester carrier, a number of advantages are realized.